1. Field of the Invention
This invention lies in the field of squelch circuits and, more particularly, radio receiver squelch circuits wherein the audio signal is used as the activating signal source.
2. Description of the Prior Art
In conventional squelch circuits, the carrier signal is used as a signal for determining when the squelch circuit is turned on or off, thus opening or closing transmission of the detected signal through the receiver to the output. In a typical prior art squelch circuit, when the AGC voltage exceeds a predetermined level set by a front panel squelch control, a squelch gate such as a transistor saturates, shunting the signal such that there is no circuit output. However, prior art devices using a carrier to noise type squelch inherently contain certain problems and, in particular, have a limitation in the lowest level of signal to noise at which the circuit may be set for squelch.
Specifically, in circuits using a carrier to noise squelch, there is a problem regarding transmission of the audio to the output with very low signal to noise ratios. In fact, intelligible audio signals can be heard with a 3 db signal plus noise to noise ratio. However, since the AGC voltage necessarily changes as the receiver gain changes, and the gain is a function of temperature, the AGC voltage is susceptible to change with temperature variations. Accordingly, circuitry using the AGC voltage to operate this squelch requires a great deal of sophisticated temperature compensation circuitry, and substantial additional gain, in order to operate the squelch at very low signal to noise levels. Although theoretically squelching can be done at desired low signal level when utilizing the AGC, this imposes substantial additional expense to ensure the required accuracy and reliability. There thus remains a great need in this field for a reliable, simple and inexpensive circuit design which will permit squelching operation at lower signal to noise levels, and which will be less sensitive to temperature variations.